When a ship moves through the water the drag resistance or water frictional forces which must be overcome are responsible for as much as half of the power consumed in operation of the vessel. The surface condition of the hull is a major factor inducing drag. It is therefore desirable to have an extremely smooth surface on the hull and paint formulations have been developed that are very smooth when cured and/or are polished by moving water to provide an extremely smooth surface. It is desirable to have a coating material that exhibits this polishing action to produce a microsmooth surface to minimize the drag penalty due to microroughness.
Fouling of the hull by pestiferous marine organisms is a major source of drag. The use of antifouling protective coatings on a ship's hull is a primary approach to controlling fouling and the resulting drag. The antifouling coating inhibits growth of marine organisms on the hull to keep it smooth. Coatings can also be used on static structures exposed to seawater to minimize growth of organisms that could cause deterioration of such structures.
A truly effective antifouling coating meets at least three criteria: (1) it will possess broad spectrum antifouling efficacy (i.e., inhibit growth of a broad variety of organisms) for extended periods of time, usually three years; (2) it will possess a smooth surface so as not to cause a microroughness drag penalty; and (3) it will actively reduce drag by reducing the roughness profile of the surface.
To meet the first criterion it is necessary to deliver to the surface of the coating in a controlled fashion, minimum effective amounts of toxin or fouling control agents. The amount of toxin delivered at the surface should not be substantially above the minimum effective amount for inhibiting fouling to avoid premature depletion of the antifouling agent.
One technique for controlling release of toxin involves the use of latent toxicants which are activated by an environmental or chemical trigger such as hydrolysis. This is the principle behind the operation of organotin acrylate copolymers as described in U.S. Pat. No. 3,167,473. In these materials a trisubstituted organotin moiety is chemically bonded to a macromolecular acrylate backbone. At the surface of the coating, the organotin moiety is liberated by hydrolysis as an active fouling control agent.
Upon hydrolysis of the organotin ester groups, the acrylate copolymer increases in hydrophilicity because of the incipient production of carboxylic acid groups in the chain. As a consequence, the acrylate copolymer loses integrity such that the outermost molecular layer of the acrylate "polishes" under dynamic conditions. By this ablation mechanism underlying organotin ester groups are eventually exposed and liberated at the surface of the coating. However, as a result of the turbulence requirement, the antifouling performance of organotin acrylate coplymers in static media is marginal.
Further, organotin acrylate copolymer films, while having a desirable smooth surface and exhibiting turbulent polishing properties, have poor integrity and require the addition of ablation control agents to control premature or uncontrolled dissolution. Even so, service life is a function of film thickness and to achieve targeted service life, very thick coatings must be used in multiple coats. This increases materials and application costs and fixes an upper limit on practical use life.